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PORRIMA

Follow the curve of the handle of the Big Dipper to the south as it first passes through orange Arcturus and then south of the sky's equator through blue-white Spica. Just up and to the right of Spica lies dimmer, third magnitude Porrima, Virgo's Gamma star (or Gamma Virginis). Unlike most star names, which are Arabic, this one is Latin and honors a Roman goddess of prophecy. A telescope shows a remarkable sight, one of the finest double stars in the sky. The components are almost perfect identical twins, both white stars with surface temperatures of about 7000 degrees, significantly warmer than the Sun. They orbit each other on highly elliptical paths in only 170 years, and as a result, a single observer can watch them easily move over the course of a lifetime. They are now about 3 seconds of arc apart, and will make their closest approach to each other in the year 2007. Thirty- eight light years away, the stars average 40 astronomical units from each other, about the distance between the Sun and Pluto. Both stars, like the Sun, belong to the "main sequence," that is, they radiate as a result of the fusion of internal hydrogen into helium. They are each about 50 percent more massive than the Sun, which results in their higher surface temperatures and in luminosities about four times solar. These otherwise ordinary stars are close to two important transition points. First, cooler stars, including those like the Sun, rotate slowly. But just cooler than the Porrima pair, stars begin to spin much faster. In the 20,000 degree range, they can rotate so fast that they come close to breaking up. Cooler stars possess the structures needed to create magnetic fields that provide the means of braking them down; warmer stars do not. Second, cooler, lower mass stars run mostly on a nuclear reaction chain that for the most part simply slams protons together to make helium. At Porrima's twin masses, a much more vigorous "carbon cycle" takes over that uses carbon to aid in the creation of helium from hydrogen.